223 research outputs found
Microwave spectroscopy of a carbon nanotube charge qubit
Carbon nanotube quantum dots allow accurate control of electron charge, spin
and valley degrees of freedom in a material which is atomically perfect and can
be grown isotopically pure. These properties underlie the unique potential of
carbon nanotubes for quantum information processing, but developing nanotube
charge, spin, or spin-valley qubits requires efficient readout techniques as
well as understanding and extending quantum coherence in these devices. Here,
we report on microwave spectroscopy of a carbon nanotube charge qubit in which
quantum information is encoded in the spatial position of an electron. We
combine radio-frequency reflectometry measurements of the quantum capacitance
of the device with microwave manipulation to drive transitions between the
qubit states. This approach simplifies charge-state readout and allows us to
operate the device at an optimal point where the qubit is first-order
insensitive to charge noise. From these measurements, we are able to quantify
the degree of charge noise experienced by the qubit and obtain an inhomogeneous
charge coherence of 5 ns. We use a chopped microwave signal whose duty-cycle
period is varied to measure the decay of the qubit states, yielding a charge
relaxation time of 48 ns
Interplay of charge and spin coherence in Landau-Zener-St\"uckelberg-Majorana interferometry
We study Landau-Zener dynamics in a double quantum dot filled with two
electrons, where the spin states can become correlated with charge states and
the level velocity can be tuned in a time-dependent fashion. We show that a
correct interpretation of experimental data is only possible when finite-time
effects are taken into account. In addition, our formalism allows the study of
partial adiabatic dynamics in the presence of phonon-mediated hyperfine
relaxation and charge-noise-induced dephasing. Our findings demonstrate that
charge noise severely impacts the visibility of
Landau-Zener-St\"uckelberg-Majorana interference fringes. This indicates that
charge coherence must be treated on an equal footing with spin coherence.Comment: 13 pages, 9 figure
High fidelity all-optical control of quantum dot spins: detailed study of the adiabatic approach
Confined electron spins are preferred candidates for embodying quantum
information in the solid state. A popular idea is the use of optical excitation
to achieve the ``best of both worlds'', i.e. marrying the long spin decoherence
times with rapid gating. Here we study an all-optical adiabatic approach to
generating single qubit phase gates. We find that such a gate can be extremely
robust against the combined effect of all principal sources of decoherence,
with an achievable fidelity of 0.999 even at finite temperature. Crucially this
performance can be obtained with only a small time cost: the adiabatic gate
duration is within about an order of magnitude of a simple dynamic
implementation. An experimental verification of these predictions is
immediately feasible with only modest resources
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